This invention relates generally to television tuning systems and particularly to television tuning systems for tuning digital television signals. More specifically it relates to double conversion type television tuning systems for use with digital television signals.
There has been recent emphasis on developing digital transmission techniques for a new high definition television system for the United States. A major shortcoming of the digital signal format is the lack of redundancy in the transmitted data, requiring that the data be very accurately detected. In analog television signals, many types of distortion and noise can be tolerated because the signal data or information is redundant and errors can, to a large extent, be integrated out. Digital data, however, has no redundancy and failure to properly detect and decode the digital data results in the information represented by that data being lost. It is therefore very important that digital signal processing utilizes suitable components prior to detection of the digital data, to assure that the RF signal carrying the digital data is not corrupted in any way.
It is also important to assure that the tuning system is not subjected to signal overloads. In conventional analog tuning systems, an automatic gain control (AGC) circuit is used as a means to reduce the tuner signal amplitude to prevent signal overload. These AGC circuits do not, by and large, meet the needs of the digital environment. For example: the minimum attenuation level should be a small fraction of 1 dB; the input and output impedances of the AGC circuit should remain well-matched with the source and load impedances at all AGC levels; the AGC range should be sufficiently large (greater than 40 dB) for very high frequencies of one gigahertz and greater; the AGC circuit should not cause any greater distortion than the other parts of the tuning system; the tuner in-band frequency response should be independent of AGC level; and the AGC regulation characteristic should be nearly linear or at least not have abrupt changes.
With the present invention, an improved AGC circuit is provided that enables control of the RF signal level applied to the IF amplifier while meeting the above-mentioned requirements.